1. Field of the Invention
The present invention relates to an imaging device, an image reading apparatus, an image forming apparatus, and an image reading method.
2. Description of the Related Art
A typical image reading apparatus that reads an image of a document or the like includes a CCD (charge coupled device) image sensor and an AFE (analog front end) arranged downstream of the CCD image sensor. An output of the CCD image sensor and an input of an AFE signal are AC (alternating current)-coupled (via a capacitor connected in series), so that only an amount of change (AC) in the output of the CCD image sensor is transferred to the AFE. In other words, the amount of change in the output of the CCD image sensor, that is, an output fluctuation amount, is inevitably transferred to the AFE.
For example, the CCD image sensor accumulates charges in response to incident light regardless of whether a power is on or off. The charge accumulation is a phenomenon that occurs not only when light is purposely applied but also when a small amount of surrounding light is detected. Therefore, when the power of the CCD image sensor is switched from off to on, unnecessary charges accumulated while the power has been off is discharged at once to an output terminal just after the power is turned on, resulting in an overvoltage.
In what is called a 4-line CCD that includes three CCD image sensors for reading color images of R (red), G (green), and B (Blue) and one CCD image sensor for reading a monochrome (BW: black and white) image, an overvoltage may sometimes occur even when the power is not switched from off to on.
For example, when the power of the 4-line CCD is on, and if a color mode for reading a color image and a monochrome mode for reading a monochrome image are switched from one to the other, unnecessary charges accumulated in the CCD image sensor that has not been used (in particular, the one CCD image sensor for reading a monochrome image is not used in the color mode and the three CCD image sensors for reading color images are not used in the monochrome mode) are discharged at once to the output terminal just after the mode is switched, resulting in an overvoltage (see FIG. 7).
If the CCD image sensor temporarily causes an overvoltage that exceeds the rated input voltage of the AFE, the overvoltage is directly transferred to the AFE and may cause characteristic degradation of the AFE device or, in the worst case, cause device breakage. Therefore, some overvoltage preventing means for the AFE have been proposed.
For example, Japanese Patent Application Laid-open No. 2006-314039 discloses a technology, in which a clamp circuit unit that fixes an input offset level at an arbitrary potential by charging or discharging an AC coupling capacitor is provided in an input unit of an analog signal processing circuit unit that is connected to a photoelectric conversion element via the AC coupling capacitor, and a period during which the clamp circuit unit fixes the input offset level at the arbitrary potential is set such that the period employed when the photoelectric conversion element is switched between on and off becomes longer than the period employed when the photoelectric conversion element is not switched between on and off.
However, in the conventional technology, it is only possible to prevent an overvoltage that occurs because unnecessary charges accumulated while the power of the CCD image sensor has been off are discharged at once just after the power of the CCD image sensor is turned on. For example, in what is called a 4-line CCD including color (R/G/B) sensors and a monochrome (BW) sensor, even when the power of the CCD image sensor is maintained in the on state, if the color reading mode and the monochrome reading mode are switched from one to the other, unnecessary charges accumulated in the sensor that has not been used (for example, the monochrome sensor in the case of the color reading mode and the color sensors in the case of the monochrome mode) are discharged at once just after the reading mode is switched, resulting in an overvoltage, which is a problem.
Therefore, there is a need for an imaging device, an image reading apparatus, an image forming apparatus, and an image reading method capable of preventing an overvoltage.